1. Field of the Invention
This invention relates to an intake system for a multiple-cylinder engine, and more particularly to an intake system for a multiple-cylinder engine which is arranged to be supercharged by a resonance tuning effect of intake air.
2. Description of the Prior Art
There have been known various multiple-cylinder engines which are arranged to increase the volumetric efficiency, thereby increasing the engine output torque, by kinetic effect of intake air. For example, in the multiple-cylinder engine disclosed in Japanese Patent Publication No. 60(1985)-14169, the cylinders are divided into first and second cylinder groups so that the cylinders in each cylinder group do not fire one after another, and the cylinders in the first and second cylinder groups are respectively connected to first and second intake passages each of which comprises an enlarged volume chamber to which discrete intake passages are connected at the upstream side ends and a resonance intake passage connected to the enlarged volume chamber. The upstream end of each resonance intake passage is communicated with an upstream side junction, and the enlarged volume chamber is provided with a switching means for selectively bringing the first and second intake passages into communication with each other and interrupting communication therebetween. When communication between the first and second intake passages is interrupted, negative pressure waves generated by the intake strokes in each cylinder are reflected at the upstream side junction into positive pressure waves, and an inertia supercharging effect is obtained by the positive pressure waves in a relatively low engine speed range. On the other hand, when the first and second intake passages are communicated with each other, the negative pressures are reflected nearer the intake port than when communication between the intake passages is interrupted, whereby the natural frequency of the intake pressure oscillation is increased, and an inertia supercharging effect is obtained in a high engine speed range.
In Japanese Unexamined Patent Publication No. 59(1984)-565, it is proposed to supercharge an engine with a compact intake manifold by providing between left and right cylinder banks of a V-type engine a plenum type intake manifold which has an outer wall having an internal space and a plurality of ram type intake pipes disposed in the internal space of the outer wall to annularly extend along the outer wall and connected to the respective intake ports at the downstream ends.
Further, supercharging can be effected by resonance tuning effect by connecting the cylinders in each cylinder group (the cylinders in which do not fire one after another) to an intake passage having no enlarged volume chamber like a surge tank and by selecting the length of the intake passage so that the resonant frequency of intake air in the intake passage conforms to a particular engine speed range.
When supercharging an engine utilizing resonance of intake air in the resonance intake passage, the resonance frequency of intake air is determined mainly depending upon the effective length of the resonance intake passage. Accordingly, in order to obtain an excellent supercharging by resonance tuning effect of intake air in an low engine speed range, the intake passage must be long, which is not preferable in view of making the engine compact.
The supercharging effect can be obtained in a wide engine speed range from the low engine speed range to the high engine speed range by providing a plurality of resonance intake passages having different effective lengths and switching the resonance intake passages according to the engine speed. However, though peaks of engine output torque can be obtained in certain engine speed ranges corresponding to the lengths of the resonance intake passages, the engine output torque cannot be increased in the remaining engine speed range and accordingly, engine output torque properties in which the torque smoothly increases from the low engine speed range to the high engine speed range cannot be obtained.